In the 1990s, the practical application of MR (magnetoresistive effect) heads and GMR (giant magnetoresistive effect) heads triggered a dramatic increase in the recording density and recording capacity of HDD (hard disk drive). However, in the early 2000s, the problem of thermal fluctuations in magnetic recording media became manifest, and hence the increase of recording density temporarily slowed down. Nevertheless, perpendicular magnetic recording, which is in principle more advantageous to high density recording than longitudinal magnetic recording, was put into practical use in 2005. It serves as an engine for the increase of HDD recording density, which exhibits an annual growth rate of approximately 40% these days.
Furthermore, the latest demonstration experiments have achieved a recording density exceeding 400 Gbits/inch2. If the development continues steadily, the recording density is expected to realize 1 Tbits/inch2 around 2012. However, it is considered that such a high recording density is not easy to realize even by using perpendicular magnetic recording because the problem of thermal fluctuations becomes manifest again.
As a recording technique possibly solving the above problem, “radio frequency magnetic field assisted recording” is proposed (e.g., U.S. Pat. No. 6,011,664). In radio frequency magnetic field assisted recording, a radio frequency magnetic field near the resonance frequency of the magnetic recording medium, which is sufficiently higher than the recording signal frequency, is locally applied to the medium. This produces resonance in the medium, which decreases the coercivity (Hc) of the portion of the medium subjected to the radio frequency magnetic field to less than half the original coercivity. Using this effect, by superposition of a radio frequency magnetic field on the recording magnetic field, it is made possible to achieve magnetic recording onto a medium having higher coercivity (Hc) and higher magnetic anisotropy energy (Ku). However, this technique disclosed in U.S. Pat. No. 6,011,664 uses a coil to generate a radio frequency magnetic field. Hence, it is difficult to efficiently apply a radio frequency magnetic field to the medium.
Thus, as a means for generating a radio frequency magnetic field, techniques based on a spin torque oscillator are proposed (e.g., United States Patent Application Publication No. 2005/0023938A1; United States Patent Application Publication No. 2005/0219771A1, and IEEE TRANSACTION ON MAGNETICS, VOL. 42, NO. 10, PP. 2670, “Bias-Field-Free Microwave Oscillator Driven by Perpendicularly Polarized Spin Current” by Xiaochun Zhu and Jian-Gang Zhu). In the techniques disclosed therein, the spin torque oscillator comprises a spin injection layer, an intermediate layer, a magnetic layer, and electrodes. When a DC current is passed in the spin torque oscillator via the electrodes, the spin torque generated by the spin injection layer produces ferromagnetic resonance in the magnetization of the magnetic layer. Consequently, a radio frequency magnetic field is generated from the spin torque oscillator.
Because the spin torque oscillator has a size of approximately several ten nanometers, the generated radio frequency magnetic field is localized within approximately several ten nanometers around the spin torque oscillator. Furthermore, the perpendicularly magnetized medium can be efficiently resonated by the longitudinal (in-plane) component of the radio frequency magnetic field, enabling a significant decrease in the coercivity of the medium. Consequently, high density magnetic recording is performed only in a portion where the recording magnetic field of the main magnetic pole overlaps the radio frequency magnetic field of the spin torque oscillator. This enables use of media having high coercivity (Hc) and high magnetic anisotropy energy (Ku). Thus, the problem of thermal fluctuations in high density recording can be avoided.
In radio frequency magnetic field assisted recording, it is important to bring the spin torque oscillator close to the main magnetic pole so that the in-plane radio frequency magnetic field and the recording magnetic field are efficiently superposed in the medium. Furthermore, it is also important to make the oscillation frequency of the spin torque oscillator nearly equal to the medium resonance frequency. However, if the spin torque oscillator is brought close to the main magnetic pole, then at a time of writing, a high magnetic field of several kOe to 20 kOe is applied from the main magnetic pole to the spin torque oscillator. Thus, the oscillation frequency and generated magnetic field intensity of the spin torque oscillator are varied with the direction of the writing magnetic field (recording magnetic field). Hence, it is required to provide a magnetic head including a spin torque oscillator capable of canceling the influence of the magnetic field generated from the main magnetic pole and a magnetic recording apparatus based on the magnetic head.
To solve this problem, it is expected to provide a technique using a spin torque oscillator in which the magnetic field applied from the main magnetic pole to the spin torque oscillator is always parallel to the magnetization of the spin injection layer when no current is passed. If a current is passed in this spin torque oscillator in the perpendicular-to-plane direction from the spin injection layer to the oscillation layer, then irrespective of the direction of the recording magnetic field, the magnetization of the oscillation layer receives a spin torque from polarized electrons reflected at the spin injection layer and undergoes precession. Irrespective of the direction of the recording magnetic field, the spin torque applied to the oscillation layer and the effective magnetic field of the oscillation layer are equal in magnitude and balanced with each other. Hence, even if the direction of the recording magnetic field is inverted, it can be expected that the oscillation frequency and generated magnetic field intensity of the spin torque oscillator are left unchanged.
On the other hand, medium inversion in each bit needs to occur while the writing magnetic field from the recording head is applied to the bit. When the writing magnetic field is inverted, the spin injection layer first starts inversion. Inversion of the spin injection layer allows the magnetization of the oscillation layer to effectively receive the spin torque of the spin injection layer, and oscillation return is started. In order to finish the oscillation return before the writing magnetic field moves to the next bit, it is necessary to shorten the sum of the inversion time of the writing magnetization, the inversion time of the magnetization of the spin injection layer, and the oscillation return time of the magnetization of the oscillation layer.
Furthermore, in the technique for passing a current in the spin torque oscillator in the perpendicular-to-plane direction from the spin injection layer to the oscillation layer, the magnetization of the spin injection layer receives a spin torque from polarized electrons passed through the oscillation layer. Hence, magnetization of the spin injection layer is made unstable. As a result, there is a problem of the decrease of spin injection efficiency of the oscillation layer.